A re-examination of the B0 and BS temperatures of steel

A new explanation of the physical significance of the B0 (the theoretical upper temperature limit) and the BS (the practical upper temperature limit) temperatures in steel is proposed. It is found that the B0 temperature of the bainite reaction in steel should be defined by the point of intersection between the thermodynamic equilibrium curve of austenite→ferrite transformation by coherent growth (curve Zγ→α→) and the thermodynamic equilibrium curve of austenite→cementite transformation (curve ES in the Fe–C phase diagram). The BS temperature for the bainite reaction is approximately 50∼55°C lower than the B0 temperature. Using this method, the BS and B0 temperatures for plain carbon steel were found to be 630°C and 680°C, respectively. The effects of steel composition on the BS temperature are discussed. The formula BS(°C)=630−45 Mn−40 V−35 Si−30 Cr−25 Mo−20 Ni−15 W can be used to predict the BS temperature of a wide range of commercial steels.